The nature and range of effects of soluble factors capable of modulating immune responses is being investigated in the mouse. Cells from mixed lymphocyte culture and/or their culture medium supernatants either suppress or stimulate in vitro antibody response to unrelated antigens. The cellular requirements for generating these activities is being studied. In addition to modulation of in vitro antibody responses, effects on a number of these in vitro and in vivo immune response models is being studied with the possible goal of being able to dissociate and manipulate humoral and cellular immunity selectivity.